Japan's Prime Minister Shinzo Abe walks after speaking on reports of the launch of a North Korean missile to reporters , at his official residence in Tokyo, Japan May 29, 2017. REUTERS/Toru Hanai

TOKYO (Reuters) - Support for Japanese Prime Minister Shinzo Abe slumped more than 10 points to 44.9 percent in a public opinion poll published on Sunday, amid opposition party suspicions he used his influence unfairly to help a friend set up a business.

Abe has repeatedly denied abusing his authority to benefit his friend. His grip on power is not in danger, given his ruling coalition's huge majority in parliament, but the affair looks unlikely to fade away.

The education ministry unearthed documents last week that the opposition said suggested Abe wanted a new veterinary school run by a friend to be approved in a state-run special economic zone. The ministry had earlier said it could not find the documents but reopened the probe under public pressure.

Opposition politicians and the media have identified Abe's friend as Kotaro Kake, the director of the Kake Educational Institution, which wants to open a veterinary department. The government has not approved new veterinary schools for decades because of concern about a glut of veterinarians.

Nearly 85 percent of voters responding to a Kyodo news agency survey said they did not think the government probe had uncovered the truth of the affair and almost 74 percent were not persuaded by the government's insistence that there was nothing wrong with the approval process.

The institution has said it had acted appropriately.

Voters were split over last week's enactment by parliament of a controversial law that will penalize conspiracies to commit terrorism and other serious crimes, with 42.1 percent in favor and 44 percent against the legislation, Kyodo said.

The government says the new legislation is needed so Japan can ratify a U.N. treaty aimed at global organized crime and prevent terrorism in the run-up to the 2020 Olympics in Tokyo.

Opponents say it will allow police to trample on civil liberties by expanding the scope for surveillance.

The ruling coalition pushed the law through parliament last week, taking the rare step of skipping a vote in committee and going directly to a full session of parliament's upper house.

Almost 68 percent of voters expressed dislike of that rarely used tactic, Kyodo said.